Determination Of Physical (K(Q)) And Chemical (K(R)) Rate Constants For Singlet Oxygen Quenching Using The Thermolysis Of A Naphthalenic Endoperoxide In H2o And D2o

A chemical source of singlet oxygen ( O-1(2) : O-2 ( (1)Delta(g))), based on thermolysis of the water-soluble naphthalenic endoperoxide NDPO2 is used to determine both the overall ( K-r + K-q) and the chemical (K-r) quenching rate constants of water-soluble substrates. Unlike current photochemical techniques, this chemical method is particularly suitable for sensitive and/or colored substrates, which can react directly with photosensitizers in their excited states or absorb incident light. This simple method has been applied to assess the reactivity of five organic compounds towards 1O2. The first one ( carminic acid) is a foodstuff coloring agent (C. I. Natural Red 4). The four other ones (methyl gallate, caffeic acid, trimethylhydroquinone, trolox) are well-known phenolic antioxidants. All of them exhibit high overall rate constants (10(7) M- (1)s(-1) < ( K-r + K-q) < 109 M-1 s(-1)) and varying range of chemical quenching (0.1 < K-r/( K-r + K-q) <= 1). The limits of the method are also discussed in detail.